Shellacs have been used as a coating for a variety of substrates for many years. Shellac is typically stored and transported in a dry, solid form under refrigeration. The particulate solid shellac is typically then solubilized in solvent, typically alcohol, prior to use. However, particulate leached shellac is quite reactive. The particulate bleached shellac is not storage stable; it has a short shelf life when exposed to ambient temperature. At ambient temperatures, particulate bleached shellac hardens within days to form a solid block; over time the shellac in the block will also begin to polymerize. If the particulate bleached shellac is exposed to heat during storage or shipping, the polymerization is accelerated.
A hardened un-polymerized or partially polymerized block of shellac may be reclaimed by grinding the block into a powder, and then solubilizing the powder in a solvent. However, the shellac solution must be strained to remove insoluble polymerized shellac solids. The straining step is a time consuming process. Moreover, if the shellac block has substantially polymerized, the shellac is no longer soluble, and even if ground, it is no longer salvageable.
Storing and shipping bleached shellac in solution, as opposed to in particulate form, does not solve the storage problems since shellac solutions also have a limited shelf life, typically one year at ambient temperature. When an aged bleached shellac solution is applied to a substrate, it will not harden into a desirable coating but instead only forms a soft, tacky film. Various compounds have been employed as additives to shellac solutions to increase shelf life. Unfortunately, such compounds typically increase the viscosity, affect the drying time, or affect the functional properties of the shellac.
It would be desirable to have a storage stable, bleached shellac which resists polymerization.
The present invention provides a bleached stable shellac and an improved method for making bleached stable shellac which involves precipitating bleached stable shellac from a pre-shellac solution at a pH of from 5.0 to 6.8, preferably from 5.0 to 6.5, most preferably from 5.0 to 6.0. As used herein, the term xe2x80x9cstable shellacxe2x80x9d refers collectively to a stable particulate shellac and a stable shellac solution made from such stable particulate shellac. The stable particulate shellac and the stable shellac solution, have a longer shelf life than conventional shellac. The stable particulate shellac may be stored longer at ambient temperatures and elevated temperatures with minimal or no polymerizing. Upon storage the stable particulate shellac is more resistant to hardening, that is, blocking, than conventional shellac. Significantly, when blocked stable shellac is ground and re-solubilized, there is little need for straining because there is minimal polymerization. The stable shellac solution is comprised of solubilized particulate shellac in a solvent, preferably a volatile organic solvent, even more preferably an alcohol, more preferably ethanol, most preferably 190 proof ethanol.
The stable particulate shellac and the stable shellac solution have a pH of from 4 up to less than 5 or from 5.0 to 6.8, more preferably from 5.0 to 6.5, even more preferably from 5.0 to 6.3, most preferably from 5.0 to 6.0. Where the de-lipidized bleached, stable shellac solution has a pH of from 5.0 to 6.8, it is characterized in that when it is heated to 100xc2x0 F. for 6 weeks, the pH drops less than 15%, more preferably not greater than 13%, most preferably not greater than 12%.
Where the lipidized, bleached stable shellac solution has a pH of from 5.0 to 6.8, it characterized in that when it is heated to 100xc2x0 F. for 6 weeks, the pH drops less than 30%, more preferably not greater than 20%, most preferably not greater than 18%.
Where the bleached stable shellac solutions are aged whether over time or in simulated conditions, the pH of both the de-lipidized and lipidized bleached stable shellac solutions tend to drop. When aged, lipidized stable shellac alcohol solution having an initial pH up to less than 5.0, is heated for 30 days at 40xc2x0 C., the pH tends to drop, to preferably not lower than 3.7, more preferably not lower than 3.8.
When aged de-lipidized stable shellac alcohol solution having an initial pH up to less than 5.0 is heated for 30 days at 40xc2x0 C., the pH tends to drop, to preferably not lower than 4.2, more preferably not lower than 4.3.
The stable shellac and an acid number of preferably from 65 to 91, more preferably 69 to 85. Once coated and dried, the aged stable shellac solution provides a harder coating than aged conventional shellac solutions.
The present invention also relates to objects coated with the stable shellac.